Primary Peritoneal Cancer: Patient Guide

The peritoneum is the tissue that lines the abdominal wall and covers the organs inside the abdomen. NCI defines primary peritoneal cancer as cancer that forms in the peritoneum and has not spread there from another part of the body.

That last clause is the whole definition. Many cancers reach the peritoneum. This one starts there.

One consequence surprises people. You can develop primary peritoneal cancer after your ovaries have already been removed. NCI notes that cancer sometimes begins in the peritoneum and spreads to the ovary, which is the reverse of what most people assume.

If your records list ovarian cancer treatment guidelines, that is not a filing error.

NCI groups three cancers together. They are ovarian epithelial, fallopian tube, and primary peritoneal. The reason is shared origin. NCI calls these tumors high-grade serous adenocarcinomas. They arise outside the uterus, from Mullerian tissue. Mullerian refers to the tissue in an embryo that forms the female reproductive tract. All three come from the same starting material, so they are staged and treated the same way.

There is a practical payoff. Since 2000, fallopian tube cancer and primary peritoneal cancer have usually been included in ovarian cancer clinical trials. When you read trial results for advanced ovarian cancer, patients with your diagnosis were generally in those trials.

The leading theory is that these cancers do not begin in the ovary at all.

Evidence came from risk-reducing surgery in healthy women carrying BRCA1 or BRCA2 variants. Pathologists kept finding small precursor lesions in the removed tissue. They sat in the fimbriae, the finger-like ends of the fallopian tubes. The lesion has a name: serous tubal intraepithelial carcinoma, abbreviated STIC.

The number attached to it is striking. NCI reports a large gap after prophylactic oophorectomy. Women with a STIC lesion were nearly 34 times more likely to develop primary peritoneal cancer than women without one. NCI draws the obvious conclusion, which is that the removed specimen needs accurate and thorough pathological review rather than a quick look.

If you had risk-reducing surgery in the past, it is worth asking whether STIC was specifically looked for and reported.

These tumors share molecular findings with ovarian cancers. Both show loss or shutdown of the tumor suppressor p53. Both can show the same for the BRCA1 or BRCA2 proteins. A tumor suppressor is a gene whose job is to stop damaged cells from dividing.

This matters for treatment selection, not just classification. It is the reason PARP inhibitors work here.

There is no early warning sign that stands out. NCI lists:

• Pain, swelling, or a feeling of pressure in the abdomen or pelvis.
• Needing to urinate urgently or often.
• Trouble eating, or feeling full quickly.
• Gas, bloating, and constipation.

NCI states plainly that these symptoms often go unrecognized, which leads to delays in diagnosis. Each one on its own reads like indigestion or aging. The pattern that should prompt an evaluation is persistence: symptoms that are new for you and that continue for weeks rather than days.

CA-125 is a protein measured in blood. It is used with the tissue findings to support the diagnosis. Treat it as one input. NCI warns that CA-125 rises in other cancers too. It also rises in benign conditions such as endometriosis.

Imaging uses several tools. They are abdominal and transvaginal ultrasound, CT, PET, MRI, and chest x-ray.

Tissue usually comes from surgery rather than a needle. NCI notes the tissue is generally removed during the operation to remove the tumor.

Surgical staging is thorough by design. In the absence of disease outside the abdomen, NCI states that definitive staging requires surgery. The surgeon first takes peritoneal washings. Then several sites get examined, biopsied, or brushed. Those sites are the diaphragm, the paracolic gutters, and the pelvic peritoneum. They also include the para-aortic and pelvic lymph nodes, and the omentum. The omentum is the fatty apron that hangs over the intestines.

Ask for the operative note. It should list each of those sites.

Stage III means the cancer has spread outside the pelvis. It has reached other parts of the abdomen, nearby lymph nodes, or both. Stage IV means it has moved beyond the abdomen, to sites such as the lungs, the liver, or distant lymph nodes.

The backbone is surgery plus platinum-based chemotherapy. The order can go either way. NCI lists surgery before or after chemotherapy. That is why some patients start with chemotherapy and have surgery partway through.

Surgery usually starts with a hysterectomy. Both ovaries and both tubes come out. So does the omentum, in a step called omentectomy.

Chemotherapy is built on two drugs, carboplatin and paclitaxel. Other options can be layered on top:

• Intraperitoneal delivery. Drug goes directly into the abdominal cavity, not only into a vein.
• HIPEC. This is heated chemotherapy placed in the abdomen during surgery. NCI lists it as an option with surgery after platinum-based chemotherapy.
• Bevacizumab. This one blocks new blood vessel growth. It can be added to induction, to consolidation, or to both.
• PARP inhibitors. These can be added to induction, to consolidation, or to both.

FDA approved olaparib with bevacizumab on May 8, 2020. It is a first-line maintenance treatment. The approval covers adults with advanced epithelial ovarian, fallopian tube, or primary peritoneal cancer. Your diagnosis is named in the approval.

Eligibility turns on a feature called HRD. The long form is homologous recombination deficiency. FDA defines HRD-positive one of two ways. A harmful or suspected harmful BRCA mutation counts. So does genomic instability. Either one, or both, qualifies. The companion test is Myriad myChoice CDx.

The trial was PAOLA-1. It enrolled 387 HRD-positive patients. Median progression-free survival was 37.2 months with olaparib plus bevacizumab. It was 17.7 months with placebo plus bevacizumab. The hazard ratio was 0.33, with a 95 percent confidence interval of 0.25 to 0.45. Olaparib is taken as 300 mg by mouth twice daily, with or without food.

This is the label's reference amount. Take what your own gynaecological oncology team prescribed, since they reduce it when blood counts fall.

NCI adds one more point. Case-control studies suggest women with BRCA1 and BRCA2 variants respond better to chemotherapy than patients with sporadic disease.

• Was HRD and BRCA testing done, on the tumor and on blood?
• If I had risk-reducing surgery before, was the specimen reviewed for STIC?
• Is the plan surgery first, or chemotherapy first with an operation in between?
• What amount of visible disease is expected to remain after surgery?
• Am I a candidate for maintenance therapy, and starting when?
• Which of my treatments were tested in trials that included primary peritoneal cancer?

• https://www.cancer.gov/types/ovarian/hp/ovarian-epithelial-treatment-pdq
• https://www.cancer.gov/types/ovarian/patient/ovarian-epithelial-treatment-pdq
• https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-olaparib-plus-bevacizumab-maintenance-treatment-ovarian-fallopian-tube-or-primary